Measuring the angle between a first member and a second member under dynamic conditions may be useful e.g. for monitoring angle variations between pivoting limbs of humans or animals, e.g. knee joints etc., under live conditions, e.g. for scientific or therapeutic reasons. Such measuring method or system has to be robust and rather insensitive for differences in the way the system is attached to those joints, e.g. by means of a flexible connection brace.
Goniometers, capable to output a measuring value in dependency of the angle between the first and second member are known per se. U.S. Pat. No. 4,542,291 discloses a goniometer, in which an optical flex sensor is provided comprising a flexible tube having two ends, a reflective interior wall within the flexible tube, a light source placed within one end of the flexible tube and a photosensitive detector placed within the other end of the flexible tube to detect a combination of direct light rays and reflected rays when the flexible tube is bent.
Other types of optical fiber type sensors for use as a goniometer are e.g. known as such from the prior art, including an imperfected graded-index. plastic optical fiber as a deformation sensor having good sensing characteristics.
This optical fiber bending sensor and other optical fiber bending sensors are also listed in an article titled A New Flexible Optical Fiber Goniometer for Dynamic Angular Measurements: Application to Human Joint Movement Monitoring by Donno, M.; Palange, E.; Di Nicola, F.; Bucci, G.; Ciancetta, F. in Instrumentation and Measurement, IEEE Transactions on Volume 57, Issue 8, Aug. 2008 Page(s):1614-1620 (reference 2 in the following). For instance, reference 2 shows the application of an optical fiber bending sensor in a joint angle sensor, the sensor being attached to an elbow by means of one or two braces.
Canadian patent application No CA 2241359 discloses a goniometer that uses angular rate sensors, such as rate gyroscopes placed on different limbs. In this case the angle is determined by integration of the angular rate. This document notes that this may lead to a drifting angle. In order to reduce drift an accelerometer is added, which is used to reset the gyroscope angle. A similar solution is described in an article titled “Detecting absolute human knee angle and angular velocity using accelerometers and rat gyroscopes”, by R. Williamson et al., published in Medical and Biological Engineering & Computing 2001, Vol. 39 pages 294-302 (EPO referenceXP001178743).
It has been found that angle measuring results from the goniometer (e.g. fiber or flexible tube type) may give less reliable measuring results due to the way it has been attached to the relevant joint, e.g. by means of one or two flexible braces. Besides that the goniometer may not have been installed quite correct initially, the goniometer (i.e. its attachments points) may, moreover, shift somewhat to and fro over the limbs, due to the flexible nature of the brace(s) and the skin of the human or annual.